1. Field of the Invention
The present invention relates to a motor driving system and, particularly, a current detecting arrangement of the motor driving system.
2. Description of the Related Art
Recently, brush-less motors are widely used because of their good durability and high performance. The brush-less motor usually has a rotor and a stator that has a plurality of phase windings. The brush-less motor is driven by the phase windings, which are energized with current that is controlled according to rotation position of the rotor. The rotation position of the rotor may be directly sensed by a position sensor such as a Hall element (sensor type) or calculated from the terminal voltage of the brush-less motor (sensor-less type).
The efficiency of the brush-less motor changes widely according to the detection accuracy of the rotation position. In case of the sensor type, the detection accuracy may become lower when a position sensor is not positioned at a right place, while the detection accuracy may become lower when motor terminal voltage abruptly changes or includes noises in case of the sensor-less type.
Because the rotor of the brush-less motor usually has a permanent magnet, the magnetic flux of the permanent magnet may be reduced or degaussed if an excessive amount of current is supplied to the motor. The degaussing increases input current in order to provide a prescribed output torque. The magnetic flux is also insufficient if the permanent magnet is not correctly magnetized during the manufacturing step of the rotor.
As shown in FIG. 9, a well-known inverter for driving a brush-less motor includes a DC power source Vs, a H-shaped bridge circuit two pair of arms 61, 62 and an electric load R and a controller. One of the arms 61, 62 is composed of an upper arm-side switch SW1 and a lower arm-side switch SW2, and the other is composed of an upper arm-side switch SW3, and a lower arm-side switch SW4. The controller controls the switches SW1–SW4 in a PWM (pulse width modulation) mode to turn on or off so as to provide an appropriate AC voltage across the electric load R.
It is well-known that an amount of phase current can be detected by a current sensing resistor element disposed between one of the lower arm-side switches SW2, SW4 and a lower voltage terminal of the DC power source. However, if the duty ratio of the lower arm-side switch SW1 or SW2 becomes less than 30%, the wave-shape of the voltage applied to the lower arm-side switch may be flattened, resulting in that the lower arm-side switch cannot turn on. Accordingly, a Hall element, which is more expensive than the current sensing resistor element, has to be used in order to detect an accurate amount of the phase current.
JP-A 2001-8488 discloses an abnormality detecting device for a brush-less motor that detects an abnormality of a brush-less motor by current supplied to a phase-winding of the motor. That is, if the current supplied to the phase winding of the brush-less motor is too small to turn on the lower arm-side switch, the amount of the current is detected from current supplied to other phase windings.
JP-A 2003-164159 discloses a current detecting device, which is not used for detecting abnormality of a brush-less motor. Even if such a current detecting device is combined to the abnormality detecting device disclosed in JP-A 2001-8488, an abnormality of the brush-less motor may not be detected unless the amount of current supplied to the brush-less motor becomes larger than a predetermined amount.
For example, if a short-circuiting takes place between a current sensor 9 and one of terminals 308, 309, 310, in a motor driving system shown in FIG. 3, little short-circuit current flows to one of current sensors 314, 315, 316. Thus, it is difficult to detect an abnormality of the brush-less motor.